1. Field of the Invention
The present invention relates to a magnetic recording or reproducing apparatus, such as a video tape recorder (hereinafter referred to as a "VTR"), which has a rotary head drum and which is arranged to record or reproduce a signal by using a magnetic tape and, more particularly, to a reel servo system for causing a magnetic tape to travel.
2. Description of the Related Art
This kind of magnetic recording or reproducing apparatus is provided with a magnetic-tape transporting system in which recording or reproduction is carried out by causing a magnetic tape to travel at a prescribed speed from a supply reel to a take-up reel in a forward (FWD) direction while keeping the magnetic tape wrapped around a rotary head drum equipped with a magnetic head by a predetermined angle such as about 180 degrees or about 360 degrees. The magnetic-tape transporting system is capable of rewinding a wound magnetic tape from the take-up reel to the supply reel, that is, in a rearward (REW) direction. The magnetic recording or reproducing apparatus is also provided with a reel servo system for causing the magnetic tape to optimally travel by controlling the driving of the magnetic-tape transporting system.
FIG. 1 is a block diagram showing a conventional type of reel servo system. The shown reel servo system includes a supply reel S, a take-up reel T, and control signal switching devices R1 and R2. The shown setting of the control signal switching devices R1 and R2 represents a magnetic tape 2 being made to travel in the FWD direction. The reel servo system also includes a rotary head drum 1 equipped with a magnetic head (not shown) for performing magnetic recording on the magnetic tape 2 or picking up a signal therefrom, a tension detecting pin 3 mounted on the entry side of the rotary head drum 1 relative to the FWD travel of the magnetic tape 2, the pin 3 being arranged to move its position in accordance with the tension of the magnetic tape 2, a tension detecting pin 4 mounted on the exit side of the rotary head drum 1 relative to the FWD travel of the magnetic tape 2, a Hall element 5 for measuring the value of a tension which is applied to the magnetic tape 2 from a position to which the pin 4 has been moved, a Hall element 6 for measuring the value of a tension which is applied to the magnetic tape 2 on the entry side of the rotary head drum 1, a timer roller 7 for outputting a magnetic-tape travel signal while it is rotating with the travel of the magnetic tape 2, a pulse interval measuring device 8 for measuring the pulse interval of a magnetic-tape travel signal outputted from the timer roller 7, a coefficient multiplier 9 for multiplying input data by its coefficient and outputting the result, a loop filter 10 for stabilizing control provided over the travelling speed of the magnetic tape 2, a coefficient multiplier 11, a loop filter 12 for stabilizing control provided over the tension of the magnetic tape 2 on the side of the supply reel S, a coefficient multiplier 13, a loop filter 14 for stabilizing control provided over the tension of the magnetic tape 2 on the side of the take-up reel T, a driver 15 for driving a reel motor 17 coupled to the supply reel S, a driver 16 for driving a reel motor 18 coupled to the take-up reel T, the reel motor 17 for driving the supply reel S, the reel motor 18 for driving the take-up reel T, analog-to-digital (A/D) converters 19 and 20, and digital-to-analog (D/A) converters 21 and 22.
In the arrangement shown in FIG. 1, while the magnetic tape 2 is being made to travel in the FWD direction, the position of the pin 3 which is made to move by a tension applied to the magnetic tape 2 on the entry side of the rotary head drum 1 is detected on the basis of an output voltage from the Hall element 6. The output voltage from the Hall element 6 is converted into digital data by the A/D converter 19, and the digital data is compared with a data value corresponding to a reference tension and the result is outputted as error data. The multiplier 11 multiplies the output error data by its coefficient and inputs the result to the loop filter 12. The output data from the loop filter 12 is converted into an analog signal by the D/A converter 21 and inputted to the driver 15 for the supply reel S. The reel motor 17 coupled to the supply reel S is controlled in accordance with the output from the driver 15 so that the tension applied to the magnetic tape 2 between the rotary head drum 1 and the supply reel S is maintained at a reference value.
The reel motor 18 coupled to the take-up reel T is placed under speed control using the magnetic-tape travel signal supplied from the timer roller 7. More specifically, the pulse interval measuring device 8 measures the period of a signal outputted from the timer roller 7 to provide magnetic-tape speed data. The speed data is compared with reference speed data to obtain speed error data. The coefficient multiplier 9 multiplies the speed error data by its coefficient and supplies the result to the loop filter 10. The data signal passed through the loop filter 10 is converted into a analog signal by the D/A converter 22. The converted analog signal is supplied as a control signal to the driver 16 coupled to the take-up reel T, thereby providing speed control over the reel motor 18 coupled to the take-up reel T.
If the magnetic tape 2 is made to travel in the REW direction, the control signal switching devices R1 and R2, respectively, are switched correspondingly so that the reel motor 17 coupled to the supply reel S which is operative as a tape receiving side is placed under speed control in a manner similar to the above-described manner In the meantime, the reel motor 18 coupled to the take-up reel T which is operative as a tape sending side is controlled so that the tension of the magnetic tape 2 wrapped around the rotary head drum 1, i.e., the tension applied to the magnetic tape 2 which is measured through the pin 4 nearer to the take-up reel T, i.e., on the tape sending side, can be maintained at a reference value.
However, the above-described conventional example has a number of disadvantages. Referring to, for example, the state of travel of the magnetic tape 2 which is transported by reel rotation, while the magnetic tape 2 is being made to travel in the FWD direction for recording or reproduction, the reel motor 17 coupled to the supply reel S is controlled on the basis of the tension applied to the magnetic tape 2 on the entry side of the rotary head drum 1. While the magnetic tape 2 is being made to travel in the REW direction for rewinding, the reel motor 18 coupled to the take-up reel T which is operative as the tape sending side is controlled on the basis of the tension applied to the magnetic tape 2 between the rotary head drum 1 and the take-up reel T. In such control, the ratio of the magnetic-tape tension measured at a tape-tension measurement point to the tension of the portion of the magnetic tape 2 which is wrapped around the rotary head drum 1 varies due to a change of set tape speeds or variations in a tape speed. As a result, the tape tension of the portion of the magnetic tape 2 which is wrapped around the rotary head drum 1 becomes unstable.
Another disadvantage is that the travelling performance of the magnetic tape 2 is easily influenced by environmental conditions such as temperature and humidity.